As this type of electromagnetic clutch, there is known an electromagnetic clutch which provides a friction plate of a rotor with three rows of grooves in the radial direction, provides the armature which faces the friction plate with two grooves in the radial direction, and makes the number of poles between the rotor and armature six poles (for example, see PLT 1). The “number of poles” is the number of pole parts which are formed between the rotor (friction plate) and the armature and corresponds to the number of locations where the magnetic flux cuts across the air gap between the rotor and the armature.